The present invention relates to a hydraulic pressure control system for a cylinder cutoff device of an internal combustion engine, and particularly to techniques for electronically controlling hydraulic pressure supplied to a hydraulically-operated active/inactive valve mode switching mechanism of an engine cylinder cutoff device through which a first group of engine cylinders are cut off with at least intake valves of their engine valves, kept inactive, while a second group of engine cylinders are working.
In recent years, there have been proposed and developed various automotive engine cylinder cutoff devices in which several cylinders are cut off during idling or during low load conditions, for improved fuel economy and improved thermal efficiency. In such engine cylinder cutoff devices, at least intake valves of several cylinders are temporarily shifted to their inactive states and stopped. One such engine cylinder cutoff device has been disclosed in Japanese Patent Provisional Publication No. 5-248217 (hereinafter is referred to as xe2x80x9cJP5-248217xe2x80x9d). In the cylinder cutoff device (or cylinder stop device or engine-valve stop device) disclosed in JP5-248217, switching between active and inactive modes of each of at least intake valves, which are subjected to cylinder cutoff control, is achieved by regulating supply oil pressure fed into an oil pressure chamber defined in the associated rocker arm by means of a hydraulic pressure control valve. In order to enhance the response to a pressure rise in supply oil pressure at the early stage of shifting to the inactive valve mode, the cylinder cutoff device of JP5-248217 uses an assist oil pump connected to the hydraulic pressure control valve. The assist oil pump is driven by way of an assist oil pump cam. The number of cam portions of the assist oil pump cam is set to be greater than or equal to the number of inactive cylinders, and therefore the assist oil pump can be kept in its discharge state at any time when switching from active to inactive and pressurized oil pressure is higher than a switching requirement oil pressure needed to switching action from active to inactive can be reliably supplied, thus minimizing the response time delay in switching to the inactive mode.
In the previously-discussed cylinder cutoff device as disclosed in JP5-248217, there are two important factors affecting the engine performance, namely, one being the response to a pressure rise in supply oil pressure at the early stage of shifting to the inactive valve mode, and the other being the response to a pressure drop in supply oil pressure in the recovery period to the active valve mode. In more detail, as a matter of course, fuel injection for inactive cylinders has to be inhibited in synchronization with the mode shifting from active to inactive. Conversely when returning to the active valve mode, that is, when the inactive valve mode is released and thus cylinder cutoff control function is disengaged, fuel injection has to be restarted in synchronization with the mode shifting from inactive to active. Assuming that the response to the supply-oil-pressure drop in the recovery period is low, there is a possibility that fuel injection initiates before the engine valve subjected to cylinder cutoff control is actually activated and opened. This deteriorates exhaust emission control performance and fuel economy. When the assist oil pump is used to realize increased supply pressure and enhanced response to the supply-oil-pressure rise at the early stage of shifting to the inactive mode, the entire system construction of the valve operating system tends to be very complicated. Owing to the use of the assist oil pump, there is an increased tendency for the supply oil pressure to be kept at a relatively high level even during inactive valve mode. As a result of this, during the recovery period from inactive to active, the response time delay in recovery to the active mode, that is, the recovery time in switching from inactive to active tends to be increased or lengthened. This deteriorates or lowers the control responsiveness during the recovery period.
Accordingly, it is an object of the invention to provide a hydraulic pressure control system for a cylinder cutoff device of an internal combustion engine, which avoids the aforementioned disadvantages.
It is another object of the invention to provide a hydraulic pressure control system for an active/inactive valve mode switching mechanism of an automotive engine cylinder cutoff device, which is simple in construction and is capable of effectively enhancing the control responsiveness during shifting to an active valve mode as well as during shifting to an inactive valve mode (or to a cylinder cutoff mode) without using an assist oil pump.
The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.